Some communication systems, commonly referred to as Multiple-Input Multiple-Output (MIMO) systems, transmit and receive simultaneously using multiple transmit and receive antennas. Some MIMO systems use the multiple transmit and receive antennas to provide enhanced capacity by applying spatial multiplexing of signals. In a typical spatial multiplexing scheme, two or more signals, which carry different data streams, are transmitted simultaneously via different transmit antennas. The signals are received by a receiver having multiple receive antennas. The receiver performs joint detection of the multiple received signals.
Several methods for joint detection of spatially-multiplexed signals are known in the art. For example, Agrell et al., provide a survey of several closest-point search methods that can be used for signal detection in “Closest Point Search in Lattices,” IEEE Transactions on Information Theory, volume 48, number 8, August 2002, pages 2201-2214, which is incorporated herein by reference.
As another example, Damen et al., describe several Maximum-Likelihood (ML) decoding algorithms for Gaussian MIMO linear channels in “On Maximum-Likelihood Detection and the Search for the Closest Lattice Point,” IEEE Transactions on Information Theory, volume 49, number 10, October 2003, pages 2389-2402, which is incorporated herein by reference. Another method for decoding a spatially-multiplexed signal, and an associated receiver, are described in U.S. Patent Application Publication 2007/0268813, whose disclosure is incorporated herein by reference.
Some joint detection methods provide soft outputs. For example, PCT Application WO 2006/135141, whose disclosure is incorporated herein by reference, describes a soft output sphere decoding method for a MIMO system. The method includes the steps of detecting a ML symbol nearest to a received signal, calculating a lattice point nearest to the received signal and having a symbol bit opposite to the detected ML symbol for all bits of the received signal, and calculating a ratio between a distance from the received signal to the detected ML symbol and a distance from the received signal to the calculated lattice points for each bit.
Studer et al., describe several performance-complexity trade-offs in the implementation of soft output sphere decoders in “Soft Output Sphere Decoding: Performance and Implementation Aspects,” Proceedings of the 40th Asilomar Conference on Signals, Systems, and Computers, Pacific Grove, Calif., November 2006, pages 2071-2076, which is incorporated herein by reference.